The purpose of this project is to gain a better understanding of the natural history of plasticity within the human spinal cord following traumatic injury, paying particular attention to changes within the autonomic nervous system and how these influence the development of spasticity. A further goal is to better understand the mechanisms underlying the recovery of conduction within axons of the corticospinal tract in persons with neurologically-incomplete spinal cord injury (SCI). This information may benefit subject selection, choice of outcome measures, and interpretation of findings as new interventions for treating human SCI make their way to clinical trials. We will recruit 150 subjects who have been admitted to University Hospital in Syracuse with acute, traumatic SCI. The overall study will last 5 years. Each subject will be studied repeatedly for a period of approximately 2 years. At each evaluation, voluntary contraction ability will be assessed with manual muscle testing and surface EMG measures. Spinal cord excitability will be measured with a variety of reflexes. Applying specific sensory inputs and recording heartrate variability, blood pressure, and other measures will examine sympathetic activity. Special emphasis will be placed on establishing the time-course of autonomic dysreflexia development, a potentially-lethal consequence of cervical and high-thoracic SCI whose mechanism is poorly understood, but which we believe is due to aberrant sprouting in the spinal cord. Corticospinal tract conduction will be examined using a novel ultra high frequency transcranial magnetic stimulation device coupled with detailed analysis of single motor unit discharge properties. A slow-release formulation the K-channel blocker 4-aminopyridine will be tested for its effects on conduction in some cases.